Earth-boring tools for forming wellbores in subterranean earth formations may include cutting elements secured to a body. For example, fixed-cutter, earth-boring rotary drill bits (also referred to as “drag bits”) include cutting elements that are fixedly attached to a body of the drill bit. Roller-cone earth-boring rotary drill bits include cones that are mounted on bearing pins extending from legs of a body such that each cone is capable of rotating about the bearing pin on which it is mounted. Cutting elements may be mounted to each cone of the drill bit.
The cutting elements used in such earth-boring tools are often polycrystalline diamond compact (often referred to as “PDC”) cutting elements, also termed “cutters.” PDC cutting elements include a polycrystalline diamond (PCD) material, which may be characterized as a superabrasive or superhard material. Such polycrystalline diamond materials are formed by sintering and bonding together small diamond grains (e.g., diamond crystals), termed “grit,” under conditions of high temperature and high pressure in the presence of a catalyst material to form polycrystalline diamond. The polycrystalline diamond is frequently in the shape of a disc, also called a “diamond table.” The processes used to from polycrystalline diamond are often referred to as high temperature/high pressure (“HTHP”) processes.
PDC cutting elements frequently include a substrate to which the polycrystalline diamond is secured. The cutting element substrate may be formed of a ceramic-metallic composite material (i.e., a cermet), such as cobalt-cemented tungsten carbide. In some instances, the polycrystalline diamond table may be formed on the substrate, for example, during the HTHP sintering process. In such instances, cobalt or other metal solvent catalyst material in the cutting element substrate (e.g., a metal matrix of the ceramic-metallic composite material) may be swept among the diamond grains during sintering and serve as a catalyst for forming a diamond table from the diamond grains. Powdered catalyst material may also be mixed with the diamond grains prior to sintering the grains together in an HTHP process. In other methods, however, the diamond table may be formed separately from the cutting element substrate and subsequently attached thereto.
To reduce problems associated with differences in thermal expansion and chemical breakdown of the diamond crystals in PDC cutting elements, “thermally stable” polycrystalline diamond compacts (which are also known as thermally stable products or “TSPs”) have been developed. Such a thermally stable polycrystalline diamond compact may be formed by removing catalyst material out from interstitial spaces among the interbonded grains in the diamond table (e.g., by leaching catalyst material from the diamond table using an acid). Diamond tables that have been at least substantially fully leached are relatively more brittle and vulnerable to shear, compressive, and tensile stresses than are unleached diamond tables. In addition, it may be difficult to secure a completely leached diamond table to a supporting substrate. To provide cutting elements having diamond tables that are more thermally stable relative to unleached diamond tables, but that are also relatively less brittle and vulnerable to shear, compressive, and tensile stresses than fully leached diamond tables, cutting elements have been provided that include a diamond table in which the catalyst material has been leached from only a portion or portions of the diamond table. For example, it is known to leach catalyst material from the cutting face, from the side of the diamond table, or both, to a desired depth within the diamond table, but without leaching all of the catalyst material out from the diamond table.
FIG. 1 is a simplified cross-sectional side view illustrating a cutting element 10 having some of the catalyst material leached therefrom. The cutting element 10 includes a substrate 12 and a diamond table 13. The diamond table 13 includes an unleached portion 14 and a leached portion 16, with a boundary 18 between the unleached portion 14 and the leached portion 16. The diamond table 13 may have a chamfer 20 and a cutting face 22. The interface 18 is shaped to generally correspond to the shape of the chamfer 20 and the cutting face 22. To form the partially leached cutting element 10 of FIG. 1, portions of the diamond table 13 and the substrate 12 may be masked, and the cutting element 10 may be placed in an acid bath, with the substrate 12 and a portion of the sidewall adjacent the substrate 12 masked to prevent leaching of a portion of the sidewall and acid damage to the substrate 12.
FIGS. 2A through 2C are perspective views illustrating how the cutting element 10 may appear after use in cutting a subterranean formation. A wear scar 24 (i.e., a surface formed by the removal of material of the cutting element 10) may begin to appear at an edge of the cutting element 10, beginning with the leached portion 16 of the diamond table 13 (FIG. 2A). As the wear scar 24 grows larger, some of the unleached portion 14 of the diamond table 13 may become exposed, surrounded by the leached portion 16 (FIG. 2B) in an aperture therethrough. After additional wear, the exposed part of the unleached portion 14 of the diamond table 13 may merge with the part of the unleached portion 14 exposed lower down the side surface of the cutting element 10 (FIG. 2C). As shown in FIG. 2C, protruding areas 26 of the leached portion 16 may extend toward one another within the wear scar 24, partially defining an alcove 28 of the unleached portion 14. As the wear scar 24 enlarges, the shape of the alcove 28 and the protruding areas 26 may change dramatically, altering the cutting performance of the cutting element 10. Surfaces of the leached portion 16 may be radially disconnected from one another (i.e., in a plane extending from the centerline of the cutting element 10) by a newly exposed portion of the unleached portion 14 during use.